1. Hokkaido Branch Laboratory, National Institute of Animal Health, Hitsujigaoka-4, Toyohira-ku, Sapporo, Hokkaido 004, Japan; National Institute of Animal Health, Kannondai 3-1-1, Tsukuba-shi, Ibaraki-ken 305, Japan
2. Hokkaido Branch Laboratory, National Institute of Animal Health, Hitsujigaoka-4, Toyohira-ku, Sapporo, Hokkaido 004, Japan